Linear hydraulic motors or jacks are commonly used for moving one member relative to another member. Serious problems can be encountered in the operation of such hydraulic motors as a result of impact of the piston against the end structure of the motor or impact between the members being controlled by the hydraulic motor. Such impacts can disturb work operations, cause undesirable noise and can cause damage to the motor itself or to the members operated thereby if sizable shock forces are generated. To reduce or eliminate such shock forces, hydraulic motors have heretofore been equipped with end of stroke cushioning devices which act essentially to progressively restrict fluid discharged from the motor as the piston approaches the end of travel or the members controlled thereby approach each other.
As heretofore constructed, such cushioning devices normally include a plunger which enters a cavity to establish a restrictive orifice a predetermined distance prior to the hydraulic motor reaching its end of stroke position. The restrictive orifice restricts the flow of fluid discharged from an actuating chamber through an inlet/outlet port of the hydraulic motor until the end of stroke is reached. One of the problems encountered therewith is that once the restrictive orifice is established, it also restricts the passage of actuating fluid from the inlet/outlet port into the actuating chamber until the plunger separates from the cavity. Such restriction slows the response time of the motor when it is actuated in the opposite direction. Another problem encountered with such cushioning devices is that they are not readily adaptable to a telescopic type hydraulic motor commonly used as the hoist motor for raising a truck body relative to the frame. The telescopic motors normally are extended by directing hydraulic fluid through a passage in the piston rod and conversely are retracted by directing the fluid discharged from the actuating chamber through the passage in the piston rod.
Thus, it would be desirable to provide a snubber or cushioning device having good snubbing capabilities at the end of stroke of the hydraulic motor but minimizes the restriction to fluid flow entering the actuating chamber to actuate the motor in the reverse direction and is suitable for use on a telescopic hydraulic motor.
The present invention is directed to overcoming the problems set forth above.